Only automobiles were issued the blue plate. Trucks, Motorcycles, and trailers began using renewal decals with the green 1975 plates.
Renewal Decals: The Truck/Trailer decal colours lagged a year behind the automobile decal colours. That is, the 1976 truck decal is the same as the 1977 car decal, and so on. If you're still confused, refer to the color chart below.